Coherent anti-Stokes Raman spectroscopy (CARS) is a known technique for investigating the properties of materials such as biological cells. In a typical CARS system, a sample is illuminated with a pump beam and a Stokes beam and responds by emitting anti-Stokes radiation. A brief, classical (as opposed to quantum-mechanical) description of the physics of CARS will now be given.
Consider a molecule having a vibrational mode with a resonant frequency of ωv. If the frequencies of the pump beam ωp and the Stokes beam ωs are such that ωp−ωs=ωv, then the molecule will respond by emitting radiation forming a CARS beam at frequency ωc=ωp+ωv that can then be detected.
Existing CARS systems utilise separate pulsed lasers to provide the pump and Stokes beams. These beams must be aligned optically and made incident upon the same volume within the target sample and the pulses from the two lasers must arrive at that volume at the same time. CARS systems of this type require expert attention in order to achieve the aforementioned spatial and temporal alignment of the delivery of the laser radiation and are often fragile in that this alignment can easily be upset (e.g., by physical shock). However, it is known to use a single laser source to provide both the pump and Stokes beams, as reported in for example in Physical Chemistry Chemical Physics 10, 609 (2008) and the documents referenced therein.
In order to target a vibrational mode of interest within a sample under analysis that has a resonant frequency ωv, the pump and Stokes beams must be tuned accurately to achieve ωp−ωs=ωv. Typically, this tuning is achieved by using diffraction gratings and liquid crystal arrays or similar to select desired probed CARS frequencies from broadband laser emissions. Again, such tuning arrangements can be awkward and sensitive to disruption.